The RIS-aided wireless networks have aroused much research interest
due to their potential benefits in 5G and beyond networks. Specifically,
the joint beamforming and reflecting design with the assistance of
RIS has received considerable attention in areas like multi-user multiple-input
single-output (MISO) downlink communication systems [1]},
multigroup multicast MISO communication systems [2]},
and multicell multiple-input multiple-output (MIMO) communication
systems [3]}, etc. While most of the existing
works have focused on single RIS assisted systems, the benefits of
designing double-RIS assisted multi-user MIMO systems cooperatively
have been explored recently in [4]}. More generally,
multi-hop RIS-aided systems can also be deployed as in the classical
multi-hop relaying systems [5]}, [6]} to
combat the propagation distance problem and to improve the coverage
range.
